Thermal transfer recording involves the formation of an image on a receptor by the transfer of a heat-activated, image-forming material from a donor element. Thermal transfer recording includes both mass transfer and diffusion transfer systems. In a mass transfer system the image is formed by the transfer of a colorant to a receptor without the occurrence of a chemical reaction. In a diffusion transfer system, the image is formed on the receptor as a result of the transfer of a chemical reactant from the donor with subsequent reaction with a coreactant on the receptor.
In each system, transfer is achieved by image-wise heating a donor sheet bearing an image-forming material. A thermal print head, which consists of an array of small, electrically heated, elements each of which is preferably computer activated in a timed sequence, is used to produce the desired image. The donor sheet typically comprises a paper or polymer film backing layer having a heat-activated, image-forming layer on its front or top surface.
In the thermal transfer process, the image-forming layer of the donor sheet is usually placed into intimate contact with a receptor surface. The back or opposite side of the donor is contacted to the thermal printhead and the printhead activated to selectively heat the image forming material and transfer it to the receptor. In this process, the donor may be exposed to temperatures of 300.degree. C. or higher for short periods of time in order to cause transfer.
Regardless of the system used to bring about transfer, it is generally the case that such material must be carried on a backing. Contact of the backing to the thermal printhead however, has been found to cause a number of problems. For example, contact can abrade the thermal printhead. Moreover, many of the commonly used backing materials are thermoplastic and have a tendency to soften and stick to the printhead during the imaging step. Each of these factors can reduce the efficiency and accuracy of the elements and cause poor print quality.
A wide variety of solutions to these problems have been suggested. They include, for example, the use of heat resistant materials as the backing material and the use of non-adhesive or anti-stick layers on the side of the backing contacting the printhead. For example, backings having softening temperatures higher than those encountered by the donor in the printing process are disclosed in unexamined Japanese patent application J6 1248-093-A, wherein copolymers containing acrylonitrile are proposed. Alternatively, materials that remain non-adhesive even though they may be softened by the heat of the printer are disclosed as anti-stick layers in unexamined Japanese patent application J8 0210-494-A, wherein polyethylene is proposed as a backing material. Both of these solutions suffer from high cost and limited availability of materials. Furthermore, while high softening and melting temperatures of polymers containing acrylonitrile give them improved heat resistance, this heat resistance hinders attempts to form them into film in an economically feasible manner. Even though polyethylene is more easily processed, due to its relatively low melting point of 137.degree. C., it requires special treatment to give it the mechanical properties necessary for use as a backing for a donor.
Because none of these approaches has been totally satisfactory, a need remains to provide an efficient and effective means for preventing fouling of the printhead.